Arborvitae (Thuja) are popular evergreen conifers often used for privacy screens and dense hedges in residential landscapes. Their year-round green foliage and upright growth habit make them a favored choice. However, their tendency to reach significant mature heights and widths often challenges homeowners trying to maintain a smaller size. Controlling the size of an arborvitae requires understanding its biology and committing to careful, consistent pruning.
Understanding Arborvitae Growth Habits
Arborvitae are biologically distinct from many other landscape plants, making size reduction difficult. As conifers, they grow primarily from the tips of their branches through terminal buds. This rigid growth pattern does not easily allow for new shoots to generate from older wood.
The plant’s dense, green foliage is concentrated on the outer layer of the branch. Beneath this layer is the older, woody stem that lacks dormant buds capable of sprouting new growth, a process known as “back budding.” If a pruning cut extends past the green needles into this old wood, the plant will not regenerate foliage. This results in a permanent, unsightly bare patch, so understanding this limitation is crucial for size control.
Techniques for Reducing Size
Because of the risk of cutting into old wood, maintaining arborvitae size relies on consistent maintenance pruning rather than heavy reduction. The goal is to trim the current season’s growth before it becomes woody and pushes the overall size outward or upward. This ensures that cuts are always made within the green, active growth zone.
For arborvitae planted as a formal hedge, the shearing method is used to maintain a consistent shape. Keep the base slightly wider than the top, creating a gentle taper. This shape allows sunlight to reach the lower branches, preventing the bottom of the hedge from thinning out.
For individual shrubs, the preferred method is reduction pruning. Cuts are made selectively back to a lateral branch or side shoot. This technique reduces the branch length while preserving the plant’s natural form and ensuring green foliage remains past the cut point. When reducing the overall size of a large plant, remove no more than 20 to 30 percent of the total foliage in a single season to avoid stressing the plant.
Timing Your Pruning Cuts
The timing of pruning cuts is linked to the plant’s annual growth cycle and its ability to recover. For major corrective pruning, such as removing significant height or width, the best time is late winter or very early spring while the plant is dormant. This timing allows the plant to use the energy surge from the beginning of the growing season to heal cuts and produce new growth.
For light maintenance and shaping of an established hedge, mid-summer is ideal, after the initial flush of spring growth. This light shearing tidies the appearance and encourages denser foliage. Avoid pruning heavily late in the growing season (late summer or fall). Late pruning can stimulate tender new growth that lacks time to harden off before winter, leading to tip damage.
Selecting Smaller Cultivars
The most sustainable strategy for size control is choosing a cultivar with a mature size that fits the intended space, eliminating the need for constant pruning. Genetic selection has produced many excellent dwarf and narrow varieties that naturally remain small. Planting the right variety from the start is far superior to managing an oversized plant through yearly pruning.
Examples of Smaller Cultivars
For instance, the popular ‘Emerald Green’ (Thuja occidentalis ‘Smaragd’) naturally maintains a narrow, pyramidal shape, reaching 10 to 15 feet tall and 3 to 4 feet wide. For a smaller, globe-shaped shrub, ‘Hetz Midget’ or ‘Danica’ are excellent choices, typically maturing at only 3 to 4 feet tall and wide. The ‘Techny’ arborvitae is a moderate grower, reaching 10 to 15 feet tall and 6 to 10 feet wide, much smaller than larger species. These cultivars provide the desired evergreen effect without the persistent maintenance challenges of rapidly growing, full-sized varieties.